In 3GPP RAN LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution), OFDMA (Orthogonal Frequency Division Multiple Access) is adopted as a downlink communication scheme. In 3GPP RAN LTE, a radio communication base station apparatus (hereinafter a “base station”) transmits a reference signal (RS) using predetermined communication resources, and a radio communication terminal apparatus (hereinafter a “mobile station”) estimates channels using a received reference signal to demodulate data (see Non-Patent Document 1).
Further, when a base station has a plurality of antenna ports, the base station can perform diversity transmission. 3GPP RAN LTE defines transmission weight referred to as a “precoding matrix” for achieving transmission diversity effect. A precoding matrix is determined by open loop or closed loop weight control.
Here, closed loop weight control includes dedicated beam forming that uses an eigenmode transmission technique, whereby individual signals can be transmitted and received in the same band at the same time by a plurality of eigenvectors. With dedicated beam forming, the base station multiplies transmission data by a precoding matrix in accordance with downlink channels, to perform beam forming. By this means, reception power of received data in the mobile station can improve. Here, with dedicated beam forming, the mobile station estimates channel conditions using an RS as is added to transmission data subject to beam forming and is able to demodulate data.
FIG. 1 shows a configuration of a base station having two antenna ports (2-Tx base station) assumed in 3GPP RAN LTE. Upon performing dedicated beam forming, a precoding section in the base station shown in FIG. 1 multiplies transmission data by a precoding matrix. Here, when the transmission data outputted from the mapping section is s and the precoding matrix by which the transmission data is multiplied in the precoding section is Φ, signal y transmitted from the base station is,[1]y=Φs  (Equation 1)where s is n−(n=2) dimensional vector, Φ is a matrix of (the number of antenna ports×n). Further, n is the number of layers in a signal, that is, the number of signals to be subject to space division multiplex (SDM).
Here, assuming that a channel matrix showing channel conditions between the base station and the mobile station is H, in the mobile station, it seems that transmission data s is received through an effective channel HΦ. That is, when transmission data is multiplied by precoding matrix Φ, in the mobile station, an effective channel is changed from channel matrix H (actual channel) to channel matrix HΦ multiplied by precoding matrix Φ. Accordingly, to receive transmission data s without an error, the mobile station needs to identify effective channel matrix HΦ.
In 3GPP RAN LTE, as shown in FIG. 2, common RSs (R0 and R1) determined between the base station and the mobile station in advance are transmitted from all antenna ports in the base station. In FIG. 2, the vertical axis (frequency domain) is subcarrier units, and the horizontal axis (time domain) is OFDM symbol units. One slot is formed with 7 OFDM symbols. R0 and R1 show RSs transmitted from antenna ports 0 and 1 (i.e. a first and a second antenna ports), respectively. A unit of one block surrounded by the bold line (12 subcarriers in the frequency domain and 7 OFDM symbols in the time domain) is referred to as a “resource block (RB).”
However, with dedicated beam forming, it is necessary to optimize precoding matrix Φ in accordance with channel conditions changing over time, and therefore it is difficult to define precoding matrix Φ for dedicated beam forming in advance. Accordingly, in 3GPP RAN LTE, precoding matrix Φ for dedicated beam forming is not defined, and precoding matrix Φ cannot be set between the base station and the mobile station in advance. Further, the common RSs shown in FIG. 2 are transmitted without being multiplied by precoding matrix Φ, and therefore the mobile station cannot estimate an effective channel (HΦ) using common RSs. Then, in 3GPP RAN LTE, studies are conducted for a dedicated RS for notifying an effective channel to mobile stations in a dedicated manner.
FIG. 3 shows an RS transmission method in a 2-Tx base station. As shown in FIG. 1, common RSs (R0 and R1) shown in FIG. 3 are transmitted without being multiplied by precoding matrices to the mobile station. Meanwhile, as shown in FIG. 1, dedicated RSs shown in FIG. 3 are multiplied by the same precoding matrices Φ as transmission data and transmitted to a mobile station. For example, the dedicated RS mapped in subcarrier number 2 shown in FIG. 3 is multiplied by precoding matrix Φ(2) found by actual channel matrix H(2) in the frequency of subcarrier number 2. By using dedicated RSs, the mobile station can estimate effective channel H(2)Φ(2). Accordingly, precoding matrix Φ is not set between the base station and the mobile station in advance, and, even when the base station sets up a precoding matrix, the mobile station is able to estimate an effective channel matrix to receive data.
As described above, by transmitting dedicated RSs from the base station, the mobile station can estimate effective channel HΦ. Here, precoding matrix Φ can be acquired by performing singular value decomposition of channel matrix H. Specifically, in the base station, matrix V calculated by singular value decomposition shown in the following equation is precoding matrix Φ.[2]H=UΣVH  (Equation 2)U and V are unitary matrices, and Σ is a diagonal matrix. Further, V is a square matrix (of the number of antenna ports×the number of antenna ports) of the base station. When the value of n differs from the number of antenna ports, the first n columns of V are precoding matrix Φ.
Further, transmission data y transmitted from the base station becomes Vs from equation 1, received signal HVs received in the mobile station is shown in the following equation.[3]HVs=UΣVHVs=UΣs  (Equation 3)
Here, by setting up UH for reception weight, a received signal after multiplying reception weight is shown in the following equation.[4]UHHVs=UHUΣs=Σs  (Equation 4)By this means, the mobile station is able to handle that transmission data s is received through channel Σ shown in a diagonal matrix.
Further, there is smoothing processing as processing to improve the accuracy of channel estimation in the mobile station. With smoothing processing, filtering is performed for a channel estimation result of continuous subcarriers in the frequency domain. For example, as the easiest smoothing processing, there is processing to average channel estimation results between neighboring subcarriers in the frequency domain. Smoothing processing is a method of using continuity of a channel in the frequency domain, and therefore it is known that, when smoothing processing is performed between subcarriers where channels of frequencies that are continuous in the frequency domain are not continuous, the accuracy of channel estimation deteriorates (e.g. see Non-Patent Document 2).    Non-Patent Document 1: 3GPP TS 36.211 V8.0.0 “Physical Channels and Modulation (Release 8),” September 2007 (ftp://ftp.3gpp.org/Specs/2007-09/Re1-8/36 series/36211-800.zip) Non-Patent    Document 2: H. Nishimoto, T. Nishimura, T. Ohgane, Y. Ogawa, “Pseudo Eigenbeam-Space Division Multiplexing in Frequency Selective Fading Environments,” IEICE Technical Report, RCS2006-56, 2006-6